ISO. 644] THE DURATION OF LIFE 



275 



The counting tube referred to above is a device in- 

 vented in this laboratory which we find extremely useful 

 in a great deal of the experimental work. It was devised 

 and first used in connection with studies of the growth 

 of experimental populations of DrosophUa {cf. Pearl 

 (7), and Pearl and Kelly (34)). Its construction is shown 

 in Fig. 1. 



When it is desired to count a definite number of flies 

 the small aperture a is temporarilv plugged with a bit 

 of cotton wool, the plunger P is removed from the tube 

 and flies are shaken into the counting tube by inverting 

 the open bottle containing them over the funnel mouth 

 of the counting tube. Then the plunger is inserted and 

 gently moved forward to concentrate the flies in the 

 lower end of the counting tube. Then the counting tube 

 with enough cotton around it to close up the mouth of 

 the bottle is inserted into the bottle into which it is de- 

 sired to place the counted flies and the plug removed 

 from the aperture a. Then as the flies come out of the 

 tube, one by one, through the aperture a, they are counted 

 as they pass this point, with the aid of a tally register, 

 such as is used by doorkeepers at theaters, etc. The plun- 

 ger is gently moved forward as necessary to keep up an 

 even flow of flies through the mouth of the tube. 



The ether dose used was constant for all the flies 

 throughout the experiment. The group to be etherized 

 was shaken into a clean half pint milk bottle; 5 c.c. of 

 ether was poured onto a piece of absorbent cotton fas- 

 tened to the under side of a cork stopper; the bottle with 



